Shigeru Kashihara

Path selection using active measurement in multi-homed wireless networks

The mobile Internet is built upon a number of different wireless access networks with widely varying features in terms of coverage area, bandwidth, packet loss, and delay. To move across these different networks smoothly, issues associated with the changing features need to be addressed. In this paper, a path selection method for the coverage overlap area is proposed in which the mobile host actively measures the round trip time (RTT) and bottleneck bandwidth for each path and a path is selected based on four rules.

Experimental Evaluation of Decision Criteria for WLAN Handover: Signal Strength and Frame Retransmission

In ubiquitous networks, Mobile Nodes (MNs) often suffer from performance degradation due to the following two reasons: (1) reduction of signal strength by the movement of an MN and intervening objects, and (2) radio interference with other WLANs. Therefore, handover initiation based on quick and reliable detection of the deterioration in a wireless link condition arising from the above two reasons is essential for achieving seamless handover. In previous studies, we focused on a handover decision criterion and described the problems related to the two existing decision criteria. Furthermore, we showed the effectiveness of the number of frame retransmissions through simulation experiments. However, a comparison of the signal strength and the number of frame retransmissions could not be examined due to the unreliability of the signal strength in simulations. Therefore, in the present paper, by employing FTP and VoIP applications, we compare the signal strength and the number of frame retransmissions as a handover decision criterion with experiments in terms of (1) and (2) in a real environment. Finally, we clarify the problem of the signal strength in contrast to the effectiveness of the number of frame retransmissions as a handover decision criterion.

Service-oriented mobility management architecture for seamless handover in ubiquitous networks

In ubiquitous wireless LANs, a mobile node is likely to move between many access points while using certain applications. However, in the conventional Internet architecture, an MN can never inherently avoid the degradation in communication quality during handover. To achieve seamless handover, we propose a service-oriented mobility management scheme to address application quality. In this article, we first clarify three requirements for achieving seamless handover. We then describe our concept of the service-oriented mobility management scheme, which satisfies all three requirements. Our main contribution is the proposal of a scheme of how to properly use the number of frame retransmissions as a new handover-decision criterion to accomplish seamless handover. Performance evaluations show that our proposed scheme can maintain application quality during handover

Handover management based on the number of data frame retransmissions for VoWLAN

Ubiquitous networks enabling access by mobile nodes (MNs) at any time and at any location will consist of innumerable Wireless Local Area Networks (WLANs). In such ubiquitous networks, it is very likely that an MN will traverse several WLANs, due to each having a small service area, during VoIP communication. Thus, a single communication may experience several handovers between WLANs divided into different IP subnets by Internet Service Providers (ISPs), during which the communication quality may be degraded due to packet loss. In this paper, we propose an effective handover management scheme for Voice over IP (VoIP) communication in which the quality is sensitive to packet loss. For this purpose, our handover management scheme employs a transport protocol supporting multiple connections for VoIP communication and makes handover decisions based on a new criterion: the number of retransmissions experienced by a data frame on layer 2. Furthermore, to prevent packet loss during handover and to investigate the wireless link condition, the proposed handover management scheme selects single-path transmission or multi-path transmission according to the number of frame retransmissions. Through simulation experiments, we show that the proposed handover management scheme can promptly and reliably detect the degradation in a wireless link, and can reduce packet loss during handover.

A quality-aware VoWLAN architecture and its quantitative evaluations

The deployment of wireless LAN and other communication media, such as 3G cellular, is proceeding rapidly. In the near future, multiple WLAN carriers, together with 3G cellular and other communication carriers, will provide their services in a complementary fashion. Furthermore, next-generation mobile phones, which may use voice and data services simultaneously, will connect these networks. In the present article we design and evaluate a new network architecture that ensures appropriate QoS for next-generation mobile phones. Through performance evaluations, we show that our architecture improves QoS for voice communication.

Experimental evaluation of decision criteria for WLAN handover: signal strength and frame retransmission

In ubiquitous networks, Mobile Nodes (MNs) may often suffer from performance degradation due to the following two reasons: (1) reduction of signal strength by an MNs movement and intervening objects, and (2) radio interference with other WLANs. Therefore, quick and reliable detection of the deterioration of a wireless link condition is essential for avoiding the degradation of the communication quality during handover. In our previous works, we focused on a handover decision criterion allowing MNs to maintain communication quality and stated the problems of existing decision criteria. Furthermore, we showed the effectiveness of the number of frame retransmissions through simulation experiments. However, a comparison between signal strength and the number of frame retransmissions could not be examined due to the unreliability of signal strength in simulations. Therefore, in this paper, by employing FTP and VoIP applications, we compare signal strength and the number of frame retransmissions as a handover decision criterion with experiments in terms of (1) and (2) in a real environment. Finally, we show the problems of signal strength in contrast to the effectiveness of the number of frame retransmissions as a handover decision criterion.

SOSCast: Location Estimation of Immobilized Persons through SOS Message Propagation

This paper proposes the design of SOS Cast, an application that will assist the search for immobilized persons in a disaster area. When a catastrophic disaster such as an earthquake or tsunami occurs, people may be injured and trapped in damaged buildings and debris. Note that a prompt rescue operation during the first 72 hours is critical in saving many lives. However, it is difficult for immobilized persons to ask for help because conventional communication services such as cellular phone networks are quite likely to be severely damaged. In a particular affected area, the SOS Cast propagates SOS messages from immobilized persons through a direct communication between smart phones. By collecting these SOS messages, rescuers can estimate the locations of the immobilized persons and view it on their smart phones. We have shown in our preliminary experiment within a residential area that SOS Cast is capable of estimating the location of an immobilized person based on the collected SOS messages.

Using SOS message propagation to estimate the location of immobilized persons

This demonstration presents SOSCast, an application that will assist the search for immobilized persons in a disaster area. When a catastrophic disaster such as an earthquake or tsunami occurs, people may be immobilized because of an injury or become trapped in buildings and debris. However, in such a situation, since conventional communication services are likely to be severely damaged, it would be difficult for immobilized persons to ask for rescue using their mobile phones. As a solution, we present SOSCast, which propagates SOS messages through direct communication between smartphones most especially inside the disaster area. By collecting these SOS messages, rescuers can estimate the locations of people who need help using the application. In this demonstration, we will show our prototype implementation of the SOSCast for SOS message processing and its map function to estimate the location of immobilized persons.

A Unified Handover Management Scheme Based on Frame Retransmissions for TCP over WLANs

In ubiquitous networks based on Wireless Local Area Networks (WLANs) with limited individual coverage, mobile nodes will be likely to traverse different WLANs during TCP communication. An effective handover management scheme for achieving seamless and efficient communication throughout the handover operation is therefore crucial. To achieve this, the following three requirements are essential: (i) early initiation of handover, (ii) elimination of communication interruption upon handover, (iii) selection of an optimal WLAN. The handover scheme proposed in this study employs frame retransmission over WLAN as an indicator of link degradation, and a handover manager (HM) on the transport layer obtains the number of frame retransmissions on the MAC layer using a cross-layer architecture in order to achieve (i) and (iii). Then, it also employs multi-homing in order to achieve (ii). Simulations demonstrate that the proposed scheme can satisfy all of the three requirements and is capable of maintaining TCP performance throughout the handover operation.

Handover management based on the number of retries for VoIP on WLANs

Ubiquitous networks enabling access by mobile nodes (MNs) at anytime and at any location will consist of several different WLANs, so that it is very, likely that the MN will traverse several hotspots during communication. Thus, a single communication can experience several handovers between WLANs, during which the quality of the communication may be degraded due to packet loss. In the present study, we attempt to develop an effective handover management scheme for VoIP communication in which the quality is sensitive to packet loss. For this purpose, we propose a handover management scheme that employs a transport protocol that supports multiple connections for VoIP communication and that makes handover decisions based upon a new criterion, i.e., the number of retries experienced by a data frame. The proposed handover management scheme minimizes packet loss, as well as redundant traffic due to parallel transfer of identical packets, by predicting the occurrence of packet loss in advance based on the number of retries and properly selecting a single-path or multi-path by examining the communication quality on the wireless link.